First-principles screening of novel ferroelectric MXene phases with a large piezoelectric response and unusual auxeticity

Abstract

Out-of-plane ferroelectricity in the two-dimensional (2D) regime shows great promise for applications in next-generation electronic devices but has been rarely reported. Herein, by using high-throughput search and density functional theory (DFT) calculations, three types of ferroelectric MXene phases (type-I: Nb₂CS₂ and Ta₂CS₂; type-II: Sc₂CO₂ and Y₂CO₂; and type-III: Sc₂CS₂ and Y₂CS₂) are for the first time predicted to be achievable by surface functionalization of 2D MXenes. The identified 2D ferroelectric MXenes not only demonstrate remarkable and reversible spontaneous electric polarization along both the out-of-plane and in-plane orientations but also exhibit giant out-of-plane and in-plane piezoelectric response to the external strain. More excitingly, the type-III ferroelectric MXenes possess rare out-of-plane auxeticity that can be triggered by the tensile strain along both the zig-zag and the arm-chair directions. The intriguing ferroelectricity, large piezoelectric response, and auxeticity render the ferroelectric MXenes versatile candidates for nanoscale electronic and mechanic devices.